1. Field of the Invention
The invention relates to the field of controls and monitors for oil well rod pumps and similar cyclic loads, and in particular concerns a method and apparatus for determining indirectly the beam position of a rod pump by monitoring variations in loading of the pump motor. By indirectly identifying the phase position of the pump cycle in this manner, a number of monitoring and control functions are enabled, substantially reducing the need to instrument the well and/or the pump.
2. Prior Art
Oil well walking beam pumps extract fluid from a downhole pump chamber by repeatedly raising and lowering a series of steel rods coupling the downhole pump and the surface beam pumper assembly. The repetitive raising and lowering of the steel rods causes a piston in the downhole pump assembly to pull the well fluids to the surface.
The surface beam pumper assembly typically includes a rocking beam with one end coupled to a pump motor by a crank assembly. The crank assembly has a counterweight intended to balance the loading of the motor by offsetting at least part of the weight of the pump connecting rods which are cantilevered on the opposite end of the rocking beam. Nevertheless, as the rods to the downhole pump are raised and lowered, the loading of the motor passes through a cycle during which potential energy is stored as the pump rods are lifted, and released as the pump rods are lowered.
The motor is typically an electric motor that is geared down to accommodate the relatively low frequency of the pump stroke. A three phase motor is typical. Motor and circuit protection contactor devices typically are provided for breaking the motor circuit in the event of a short circuit or motor overload. Additionally, a controller that is responsive to conditions in the well may be coupled to the contactor devices, for example to operate the pump intermittently at a rate that can be supported by the geological formation. The controller or the contactor device itself may include means for measuring the current in the motor circuit and/or the line voltage by analog or digital circuits, as a part of the circuit protection function, as well as to vary the operation of the pump to suit conditions at the best efficiency.
It is known to provide a contactor for an oil well with relay contacts that rearrange the line couplings of a three phase motor when current loading conditions indicate that the pump is operating inefficiently, for example as disclosed in U.S. Pat. No. 4,220,440--Taylor et al. U.S. Pat. No. 4,695,779--Yates discloses a similar controller that includes a processor and a number of timers that switch between operational modes upon the occurrence of distinct stall conditions.
A processor with a range of flow and energy consumption sensors for assessing well operation is disclosed in U.S. Pat. No. 4,767,280--Markuson, and a processor that integrates additional factors such as the proportions of oil and water in the recovered fluid is disclosed in U.S. Pat. No. 5,070,725--Cox et al.
Although the invention is described herein primarily with reference to a walking beam pump, it is also possible to apply the concepts of a walking beam pump to other forms of cyclic loads. U.S. Pat. Nos. 4,601,640 and 4,493,613, both to Sommer, for example, disclose a compact pump arrangement that reciprocates a piston but does not employ a beam. Instead, a reversing motor manipulates the piston via a cable. These, and the foregoing U.S. Patent disclosures are hereby incorporated by reference, for their teachings of well motor control and sensing arrangements.
Wells are frequently instrumented for purposes of assessing operational parameters. The fluid flow rate produced by the well is an advantageous parameter to measure, and can be measured using flow rate sensors at any point along the conduits through which the fluid is pumped. The fluid pressures produced in the well by the pump can also be monitored, and used to develop additional information, such as the rate at which the geological formation is refilling the pump, and other aspects of well performance. One means for sensing well fluid pressure indirectly is to sense tension and compression of the moving pump structures, for example using strain gauges mounted on such structures or load cells coupled between them.
There are a number of aspects of well and/or pump performance that are pertinent to issues of efficiency, maintenance, capacity, switching between operational modes and the like. The object for the well is of course to supply the maximum fluid possible, and preferably to maximize the percentage of the fluid that is oil rather than water or mud while minimizing the power consumption of the pump. However, optimizing pump operation requires that the operation of the pump be varied to suit conditions. A monitoring system and controller can be provided to sense conditions and to adjust operational parameters such as the frequency of cyclic operation, the manner in which power is coupled to the motor windings and so forth.
The amount of useful work that a fluid transport device performs is the product of the mass rate of fluid flow and the pressure differential or elevation head. The total head borne by the pump includes static and dynamic factors such as the discharge head and the suction head maintained, a velocity head, frictional resistance, etc. The variations in a number of these factors, especially fluid pressure and fluid flow, is cyclic due to the cyclic operation of the pump. It is therefore necessary to assess fluid pressure and flow information as a function of the point at which such data is sampled in the periodic cycle of the pump. The monitoring and control system of the pump thus requires the input of information on the present phase angle of the pump.
The phase angle of the pump can be measured by more or less sophisticated means. For example, a limit switch can be mounted for repetitive operation by contact with the pump beam, and used to trigger sampling of process data at the same point during every cycle, or between counted cycles. A shaft angle encoder can be mounted to produce pulses with angular displacement of the beam or of the motor crank, etc., which allows measurements to be taken at defined points in the cycle. These devices require proper setup and maintenance, and can suffer from mechanical failure. Thus the known arrangements are expensive both initially and with continuing maintenance and use.
It would be advantageous to provide a device that can determine information needed for assessing or controlling pump operation using a minimum of components. The present invention is arranged to develop such information indirectly from variation in the loading of the pump motor, and in particular determines a reference phase angle in the pump cycle from the point of minimum motor power that occurs at the top dead center of the pump stroke.